2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate synthase

InterPro Family 2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate synthase 4.2.99.20 122007-88-9 2-Succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate synthase (EC 4.2.99.20), also known as SHCHC synthase, is encoded by the menH gene in Escherichia coli and functions in the synthesis of vitamin K. Jiang M, Chen X, Guo ZF, Cao Y, Chen M, Guo Z Identification and characterization of (1R,6R)-2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate synthase in the menaquinone biosynthesis of Escherichia coli Biochemistry 47 11 3426–34 March 2008 18284213 10.1021/bi7023755 The specific step in the synthetic pathway that SHCHC synthase catalyzes is the conversion of 2-succinyl-5-enolpyruvyl-6-hydroxy-3-cyclohexene-1-carboxylic acid to (1R,6R)-2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylic acid and pyruvic acid. http://www.brenda-enzymes.org/enzyme.php?ecno=4.2.99.20 Information on EC 4.2.99.20 - 2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate synthase 2014-07-01 2014-11-01 Brenda: The Comprehensive Enzyme Information System TU Braunschweig Background Vitamin K is a fat soluble vitamin known to aid in blood clotting. It is recommended that all newborns receive an injection of vitamin K in order to prevent excessive bleeding of the brain after birth. There are two major forms of vitamin K that occur naturally. Phylloquinone, also known as K1, is synthesized by plants and is the major form of vitamin K in the diet. Menaquinone, K2, includes a range of forms that are synthesized by bacteria in the gut. http://lpi.oregonstate.edu/infocenter/vitamins/vitaminK/ Micronutrient Information Center 2014-11-30 2014-11-30 Linus Pauling Institute Vitamin K is synthesized from the molecule chorismate in a nine step conversion process. SHCHC synthase catalyzes the third step in the process.van Oostende C, Widhalm JR, Furt F, Ducluzeau AL, Basset GJC (2011) Phylloquinone (Vitamin K1): function, enzymes and genes. in Advances in Botanical Research, eds Fabrice Rébeillé and Roland Douce, 59: 229-61, Academic Press (Amsterdam). Chemistry Reaction scheme The previous enzyme in the biosynthetic pathway, SEPHCHC synthase produces 2-succinyl-5-enolpyruvyl-6-hydroxy-3-cyclohexene-1-carboxylic acid (SEPHCHC), Dawson A, Fyfe PK, Hunter WN Specificity and reactivity in menaquinone biosynthesis: the structure of Escherichia coli MenD (2-succinyl-5-enolpyruvyl-6-hydroxy-3-cyclohexadiene-1-carboxylate synthase) Journal of Molecular Biology 384 5 1353–68 December 2008 18983854 3656419 10.1016/j.jmb.2008.10.048 which SHCHC synthase converts to the (1R,6R) isomer of 2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylic acid (SHCHC).4.2.99.20 70% Q27122013 SEPHCHC SHCHCSynthase Q27095770 SHCHC + pyruvic acid 0.5 Enzyme structure thumb|Monomer of MenH enzyme in E.coli (SHCHC synthase). The structure is one continuous amino acid sequence, but the catalytic triad is shown as cyan spheres and the alpha helix cap is shown in red. The rest of the protein sequence is shown in green.|278x278px The crystal structure of the MenH enzyme in E.coli (SHCHC synthase) exists as a complex of three protein molecules shown in the diagram. SHCHC synthase forms an alpha/beta hydrolase fold with a central set of seven parallel beta sheets surrounded by alpha helices on both sides. A cap of five alpha helixes serves to enclose the active site.4GDM; Johnston JM, Jiang M, Guo Z, Baker EN Crystal structures of E. coli native MenH and two active site mutants PLOS ONE 8 4 e61325 2013-04-18 23637813 3630204 10.1371/journal.pone.0061325 2013PLoSO...861325J free The enzyme exists in an open form until it binds the substrate, when it morphs into a closed form with an active catalytic triad. Sun Y, Yin S, Feng Y, Li J, Zhou J, Liu C, Zhu G, Guo Z Molecular basis of the general base catalysis of an α/β-hydrolase catalytic triad The Journal of Biological Chemistry 289 22 15867–79 May 2014 24737327 4140940 10.1074/jbc.M113.535641 free Energetic analysis shows that SHCHC synthase has a low energetic burden for catalytic activity. This means the enzyme is more prone to mutation and is one of the most diverse enzymes in the vitamin K synthetic pathway. Jiang M, Chen X, Wu XH, Chen M, Wu YD, Guo Z Catalytic mechanism of SHCHC synthase in the menaquinone biosynthesis of Escherichia coli: identification and mutational analysis of the active site residues Biochemistry 48 29 6921–31 July 2009 19545176 10.1021/bi900897h Only fifteen amino acid residues are absolutely conserved across mutations of the enzyme. Catalytic mechanism The active site contains a catalytic triad of syrine, histine and arginine, which is conserved across all mutants and is proposed to initiate the reaction. The triad residues are located at Ser86, Asp210, and His232. This triad is proposed to catalyze a proton extraction which triggers a transfer of electrons leading to the elimination of pyruvate and formation of SHCHC. Originally, it was proposed that the transition state was stabilized by a nontraditional oxyanion hole. Now a traditional oxyanion hole is favored, but not definitive. Reaction mechanism centre|800x800px|frameless Cofactors and alternate reactions SHCHC synthase is unaffected by traditional cofactors such as divalent metal ions and EDTA. The enzyme is fairly specific and only acts on SEPHCHC and close derivatives. Controversy MenH (SHCHC synthase) was previously thought to be a thioesterase involved in hydrolyzing DHNA-CoA in a later step of menaquinone synthesis. In 2008, it was determined that MenH has poor catalytic activity toward palmitoyl-CoA, casting doubt on its role as a thioesterase. Direct analysis confirmed that MenH is unable to hydrolyze DHNA-CoA. In 2009, it was proposed that a dedicated hotdog fold thioesterase would be needed to catalyze the hydrolysis of DHNA-CoA. Widhalm JR, van Oostende C, Furt F, Basset GJ A dedicated thioesterase of the Hotdog-fold family is required for the biosynthesis of the naphthoquinone ring of vitamin K1 Proceedings of the National Academy of Sciences of the United States of America 106 14 5599–603 April 2009 19321747 2660889 10.1073/pnas.0900738106 2009PNAS..106.5599W http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1033&context=plantscifacpub free A protein was identified in 2013 that could fit this role. Chen M, Ma X, Chen X, Jiang M, Song H, Guo Z Identification of a hotdog fold thioesterase involved in the biosynthesis of menaquinone in Escherichia coli Journal of Bacteriology 195 12 2768–75 June 2013 23564174 3697248 10.1128/JB.00141-13 References 32em Biology no Category:EC 4.2.99 Category:Enzymes of known structure